1. Technical Field
The present invention relates to an image forming control apparatus that performs a gradation correction process based on calibration data for adjusting a density of an image for an image forming apparatus, and an image forming control method and a computer readable medium.
2. Related Art
When an image forming apparatus, such as a copier or a printer, employs image data to form an image on a recording medium, such as paper, the image forming apparatus, based on output instruction values determined in accordance, for example, with pixel values accompanying the image data, adjusts a volume of a toner or a volume of an ink to be used in order to control the density of the image to be formed, and consequently forms the image of which a gradation is accordance with the image data, on the recording medium. In order for the image forming apparatus to perform such a control process, an image forming control apparatus, which is connected to the image forming apparatus, includes a conversion table that is based on a reference gradation characteristic that represents a correspondence between pixel values (input values) of the image data, and output instruction values for reproducing target values (standard density values) for a density of the image to be formed on recording media. The image forming control apparatus employs the conversion table to convert the input values of the image data into output instruction values corresponding to the standard density values. Then, the output instruction values obtained through the conversion are output to the image forming apparatus as density instructions for the image to be formed. Based on the output instruction values, the image forming apparatus adjusts the volume of the toner, for example, to be used to form the image having a standard density that is in accordance with the input values of the image data.
However, since generally the state of the image forming apparatus fluctuates, depending on the operating environment and time-transient changes, there is a case where, even though the conversion is employed, the density of an image actually formed on a recording medium deviates from the standard density. Therefore, a technique is provided whereby an image forming control apparatus employs calibration data to adjust the density. According to this technique, for example, the image forming apparatus obtains data concerning a correspondence between an output instruction value for an image forming apparatus and the density value of an image actually formed, and based on the data, generates calibration data that specifies a correspondence between the standard density value and the output instruction value. Then, a gradation correction is performed, and the output instruction values that have been obtained are corrected, based on the above described conversion table, by converting the input values of the image data. Thereafter, to adjust the volume of the toner for the image forming apparatus, the values obtained by correction are output as corrected instruction values. Through this processing, the image forming control apparatus ensures that the image forming apparatus forms an image in accordance with the print engine characteristic of the image forming apparatus for the generation of calibration data.
FIG. 4A is a schematic graph showing an example of a correction to be performed between an output instruction value and a density value of an image actually formed by an image forming apparatus. In this graph, the horizontal axis represents an output instruction value, which falls within a range of 0 to 255, and the vertical axis represents an actual density value. As indicated by a curve C11 in FIG. 4A, in the standard state of the image forming apparatus, actual density value Dm corresponds to the maximum output instruction value 255, and is regarded as the maximum standard density value. Further, a curve C12 in FIG. 4A represents a case wherein, relative to the same output instruction value, the image forming apparatus forms an image at a density greater than that in the standard state. In this case, when the image is formed at the maximum output instruction value of 255, the actual density value exceeds the maximum standard density value Dm.
In such a case, when an output instruction value Ye that is smaller than the maximum value 255 is output, an image can be formed at the maximum standard density value Dm. Therefore, based on a correspondence represented by a curve C12, the image forming control apparatus generates a gradation correction curve C13 shown in FIG. 4B, and generates calibration data based on this curve C13. For the calibration data, the maximum output instruction value 255 is converted into an output instruction Ye (<255) corresponding to the maximum standard density value Dm. As a result, in order to ensure the image forming apparatus in the standard state forms an image at the maximum standard density value Dm, the image forming control apparatus performs the gradation correction process to convert the output instruction value 255 to the output instruction Ye. Thus, a user can obtain an image formed at the maximum standard density value Dm.